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We report observations of molecular oxygen (O$_2$) rotational transitions at487 GHz, 774 GHz, and 1121 GHz toward Orion Peak A. The O2 lines at 487 GHz and774 GHz are detected at velocities of 10-12 km/s with line widths 3 km/s;however, the transition at 1121 GHz is not detected. The observed linecharacteristics, combined with the results of earlier observations, suggestthat the region responsible for the O$_2$ emission is 9" (6e16 cm) in size, andis located close to the H2 Peak 1position (where vibrationally-excited H$_2$emission peaks), and not at Peak A, 23" away. The peak O2 column density is1.1e18/cm2. The line velocity is close to that of 621 GHz water maser emissionfound in this portion of the Orion Molecular Cloud, and having a shock withvelocity vector lying nearly in the plane of the sky is consistent withproducing maximum maser gain along the line-of-sight. The enhanced O$_2$abundance compared to that generally found in dense interstellar clouds can beexplained by passage of a low-velocity C-shock through a clump with preshockdensity 2e4/cm3, if a reasonable flux of UV radiation is present. The postshockO$_2$ can explain the emission from the source if its line of sight dimensionis ~10 times larger than its size on the plane of the sky. The special geometryand conditions required may explain why O$_2$ emission has not been detected inthe cores of other massive star-forming molecular clouds.

Herschel HIFI observations of O$_2$ toward Orion: special conditions for shock enhanced emission